1. Field of the Invention
Methane production by anaerobic digestion has been widely practiced, particularly with respect to digestion of sewage sludge organic waste. In recent times, the worldwide energy shortage has furthered consideration and improvement of non-fossil sources of energy. Biological materials, including organic wastes, represent a large renewable potential energy resource. This invention relates to improved gasification from biological materials by hybrid biological-thermal gasification to provide substantially increased conversion of the organic component of the biological feed material, including terrestrial and aquatic energy crops, organic wastes and peat. The process of this invention provides anaerobic digestion of a biological feed producing methane containing gas followed by thermal gasification of the biological residue from the anaerobic digestion, the thermal gasification providing product gas for supply to the anaerobic digester for production of methane. The thermal residue from such gasification may be recycled to the anaerobic digester to provide phosphorus and other inorganic nutrients. The hybrid bio-thermal gasification process of this invention broadens the range of biological feeds suitable for conversion, provides higher gas and methane production per pound of biological feed, and substantially reduces the quantity of residue from the process. The hybrid bio-thermal gasification process of this invention may provide medium Btu fuel gas, substitute natural gas (SNG) and hydrogen containing gas for a variety of uses.
2. Description of the Prior Art
The production of methane gas by anaerobic digestion of various organic wastes has been known. There have been continuous efforts to improve methane yield resulting from anaerobic digestion. Most of the prior attempts to increase methane yield have been centered around anaerobic digestion as practiced in municipal waste treatment plants as exemplified by U.S. Pat. Nos. 3,640,846, teaching addition of coal; 3,981,800, teaching pressurized digestion; and 4,022,665, teaching two phase digestion of sewage sludge. Other attempts to improve the production rate and yield of methane by anaerobic digestion have related to improved anaerobic digestion by utilization of liberated enzymes of the biomass for contribution to more efficient digestion as taught by U.S. Pat. No. 3,994,780.
U.S. Pat. No. 2,638,444 and U.S. Pat. No. 4,100,023 teach recycling product gases from an anaerobic digester back to the digester for the function of agitation and heat addition. However, such gases would principally contain methane and carbon dioxide which may inhibit methane production due to mass action. The shift conversion of carbon monoxide and water to carbon dioxide and hydrogen and the anaerobic production of methane from carbon monoxide, carbon dioxide and hydrogen has been reported by D. L. Wise, C. L. Cooney and D. C. Augenstein, Biomethanation: Anaerobic Fermentation of CO.sub.2, H.sub.2, and CO to Methane, Biotechnology and Bioengineering, Vol. XX, Pp. 1153-1172 (1978). The biological conversion of carbon monoxide to methane has also been reported in Pure Cultures of Methanogenic Bacteria by Daniels and Zeikus, Journal of Bacteriology, 136; 75 (1977).
The prior art, while recognizing the desirability to increase methane production resulting from anaerobic digestion, does not suggest the combination of biological and thermal gasification to utilize substantially all of the organic hydrocarbon material in the biological feed.